Pen

This is Pen, a highly scalable, highly available, robust load balancer
for tcp and udp based protocols such as
dns, http or smtp. It allows several servers to appear as one to the
outside and automatically detects servers that are down and distributes
clients among the available servers. This gives high availability and
scalable performance.

Pen works on Linux, FreeBSD, OpenBSD, NetBSD, Solaris and most other
Posix-like systems. It can also be installed as a service on Windows.

The load balancing algorithm keeps track of clients and will try to
send them back to the server they visited the last time. The client
table has a number of slots (default 2048, settable through command-line
arguments). When the table is full, the least recently used one will
be thrown out to make room for the new one.

This is superior to a simple round-robin algorithm, which sends a client
that connects repeatedly to different servers. Doing so breaks
applications that maintain state between connections in the server,
including most modern web applications.

When pen detects that a server is unavailable, it scans for another
starting with the server after the most recently used one. That way
we get load balancing and "fair" failover for free.

Correctly configured, pen can ensure that a server farm is always
available, even when individual servers are brought down for maintenance
or reconfiguration. The final single point of failure, pen itself,
can be eliminated by running pen on several servers, using vrrp to
decide which is active.

Release notes

Setting conn_max in config file or using penctl would change the size of the
client table, not the connections table as expected. See issue #38.
Full list of changes:
170217 Corrected typo in pen.c per suggestion by Belinda Liu.
This fixes issue #38.
Released 0.34.1.

Penlogd

A side-effect of the load-balancing is that several logfiles are produced,
and all accesses seem to come from the load balancer. The program
penlogd solves this problem by merging pen's log file with the ones
produced by the web servers. See penlogd(1) and penlog(1) for details.

Webstats

Pen emits statistics when it receives a USR1 signal, but the output
can be hard to interpret. The cgi script webstats
can be used in conjunction with the -w option to pen to get statistics
in HTML format instead.

Take a look here to see what
a Pen status can look like here at siag.nu. This is not "live" data, but
generated from this cron job every 10 minutes:

2,12,22,32,42,52 * * * * kill -USR1 `cat /var/run/pen.pid`

Note that there are usually few active connections. This is because of the
way the web works: the client connects, the server sends data, the
client disconnects. The whole transaction is over in seconds.

Portability

This load balancer is known to work on a long lite of operating systems,
including but not limited to FreeBSD, Linux, HP-UX, Windows and Solaris.
Other Unixes should work as well, possibly requiring trivial changes.
Success stories or problem reports are welcome.